1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly, to a structure of a charge transfer device.
2. Description of the Prior Art
A conventional example of a charge coupled type or CCD type image pick-up device is disclosed in C. H. Sequin, M. F. Tompsett "Charge Transfer Device" published by Academic Press New York, N.Y. 1975. Such a C.C.D. type image pick-up device includes, as shown in FIG. 1(a), a light receiving part 403 consisting of a photoelectric conversion region 401 and a vertical CCD region 402, a horizontal CCD part 404 and an output amplifier 405. The light receiving part 403 is shown in FIG. 1(b) in which an n.sup.+ region 407 formed on a surface of a P substrate 406 constitutes the phototelectric conversion region 401 of a p-n junction type photodiode, while the n.sup.- region 408 having a high resistivity constitutes the vertical CCD region 402 in association with transferring gate electrode 409. In attempting to make the device high in resolution by increasing the device density while maintaining the size of the area of the device as it is, (that is maintaining the optical lens system as it is), it is necessary to divide one area of one picture element or one pixel into the photoelectric conversion region 401 and the vertical CCD region 402. Since the size occupied by the photoelectric conversion region 402 (which is called as opening rate) defines the sensitivity and the size of the vertical CCD region 402, relates to the width of the charge transferring channel and defines the dynamic range, there occurs a trade off between sensitivity and dynamic range.
As a result of study of dissolving the above problem, the present inventors have found that it is possible to enlarge the effective channel width of the CCD device in such a manner that there constitutes a three dimensional trench 502 or recess in the form of a groove in the vertical CCD region 402 as shown in FIG. 2 (disclosed in the Japanese Patent Publication (unexamined) 63-12162). More specifically, an elongated trench groove 502 is formed in a substrate 501 of a P conductivity type so as to extend in one direction and an n conductivity region 503 acting as a buried channel region is formed on the surface of the trench groove 502. Thereafter, a number of first transfer electrodes 505 are formed on the surface of the trench groove 502 which are isolated by insulation layers 504 and a number of second transfer electrodes 507 are formed on the surface of the trench groove 502 which are isolated by insulation layers 506. By the arrangement mentioned as above, since the area occupied by the vertical CCD region per one pixel can be decreased, the size of the phototelectric conversion region 401 can be increased, thereby resulting in an improved sensitivity of the CCD image pick-up device. In addition, in the vertical CCD unit 402, the dynamic range can be improved by increasing the effective channel width of the transfer channel by increasing the depth of the trench groove 502. (reference literature: T. YAMADA etal, "TRENCH CCD IMAGESENSOR" IEEE 1989, International Conference on Consumer Electronics, pp 178-179, June 1989).
However, there is a specific problem of constituting the trench groove in the CCD device. Namely, the transfer electrodes 505 as shown in FIG. 2 are formed by processes of uniformly forming a poly silicon layer, on the wall surface of the trench groove and then removing unnecessary portions of the layer by etching. The difficulty of completing the transfer electrodes increases as the depth of the trench groove increases.
The CCD device with the trench groove has two kinds of transfer electrodes 505 and 507 similar to the conventional CCD device. Therefore, it is necessary to repeat the etching process twice, and it is absolutely necessary to prepare protection film for proofing the damage of the semiconductor portion due to long time duration of the etching process, whereby the resultant burden in the production process is not negligible.